The present invention relates to a process for the desulfurization of a full boiling range fluid catalytic cracked naphtha by the reaction of hydrogen with the organic sulfur compounds present in a feed in the presence of a hydrodesulfurization catalysts. In particular the present invention may employ catalytic distillation steps which reduce sulfur to very low levels, makes more efficient use of hydrogen and causes less olefin hydrogenation for a full boiling range naphtha stream.
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling range which determines the composition. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The composition of untreated naphtha as it comes from the crude still, or straight run naphtha, is primarily influenced by the crude source. Naphthas from paraffinic crude sources have more saturated straight chain or cyclic compounds. As a general rule most of the xe2x80x9csweetxe2x80x9d (low sulfur) crudes and naphthas are paraffinic. The naphthenic crudes contain more unsaturates and cyclic and polycylic compounds. The higher sulfur content crudes tend to be naphthenic. Treatment of the different straight run naphthas may be slightly different depending upon their composition due to crude source.
Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction for valuable aromatic product removal. Reformed naphthas have essentially no sulfur contaminants due to the severity of their pretreatment for the process and the process itself.
Cracked naphtha as it comes from the catalytic cracker has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. In some cases this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane.
Catalytically cracked naphtha gasoline boiling range material currently forms a significant part (≈⅓) of the gasoline product pool in the United States and it provides the largest portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations. Some users wish the sulfur of the final product to be below 50 wppm.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:
RSH+H2xe2x86x92RH+H2Sxe2x80x83xe2x80x83(1)
RCI+H2xe2x86x92RH+HClxe2x80x83xe2x80x83(2)
2RN+4H2xe2x86x92RH+NH3xe2x80x83xe2x80x83(3)
ROOH+2H2xe2x86x92RH+H2Oxe2x80x83xe2x80x83(4)
Typical operating conditions for the HDS reactions are:
After the hydrotreating is complete, the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha. The loss of olefins by incidental hydrogenation is detrimental by the reduction of the octane rating of the naphtha and the reduction in the pool of olefins for other uses.
In addition to supplying high octane blending components the cracked naphthas are often used as sources of olefins in other processes such as etherifications. The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction reducing the octane and causing a loss of source olefins.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction the most common solution has been prefractionation prior to hydrotreating. The prefractionation produces a light boiling range naphtha which boils in the range of C5 to about 250xc2x0 F. and a heavy boiling range naphtha which boils in the range of from about 250-475xc2x0 F.
The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. The separation by fractionation alone will not remove the mercaptans. However, in the past the mercaptans have been removed by oxidative processes involving caustic washing. A combination oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans the mercaptans are converted to the corresponding disulfides.
U.S. Pat. No. 5,597,476 discloses a two step process in which naphtha is fed to a first distillation column reactor which acts as a depentanizer or dehexanizer with the lighter material containing most of the olefins and mercaptans being boiled up into a first distillation reaction zone where the mercaptans are reacted with diolefins to form sulfides which are removed in the bottoms along with any higher boiling sulfur compounds. The bottoms are subjected to hydrodesulfurization in a second distillation column reactor where the sulfur compounds are converted to H2S and removed.
The use of two or more sequential catalyzed hydrodesulfurization treatments has been practiced for treating petroleum feeds to remove the organic sulfur compounds, however in these prior processes there is a significant quantity of xe2x80x9crecombinant sulfurxe2x80x9d in the product. Recombinant sulfur means new organic sulfur compounds, mainly mercaptans, that are formed by the reverse reaction of H2S from the preceding hydrodesulfurization or hydrodesulfurizations in succeeding hydrodesulfurizations with olefins in the feed. The H2S can recombine to form mercaptans thus increasing the amount of sulfur in the product. Also, the presence of H2S can cause more of the olefins to be saturated losing octane and consuming hydrogen.
The product from the sequential multibed hydrodesulfurizations does of course have lower sulfur content than a feed having less treatment, but each subsequent treatment has been less efficient than expected because of the recombinant sulfur.
In the present process the efficiency of a subsequent treatment has been increased by stripping the H2S from the treated feed prior to passing the feed to a subsequent hydrodesulfurization.
It is an advantage of the present invention that a full boiling range naphtha stream is hydrodesulfurized while the olefin content is maintained at a high percent of the original feed by sequential hydrodesulfurization with removal of H2S after each hydrodesulfurization. It is not believed that H2S has been removed between hydrodesulfurization treatments in sequential hydrodesulfurizations in the prior art, but rather the prior practice has been to remove the H2S after the final hydrodesulfurization.
In its broadest context the present invention is in a process of hydrodesulfurization of a petroleum feed containing organic sulfur compounds and preferably olefins by at least two sequential treatments of the feed by contact with hydrogen in the presence of a hydrodesulfurization catalyst to convert a portion of the organic sulfur compounds to H2S, wherein the improvement is the removal of H2S from the feed after each said treatment.
In one embodiment of the present invention a full boiling range naphtha is subjected to a two stage process for the removal of organic sulfur by hydrodesulfurization. In the first stage the full boiling range naphtha is desulfurized in a distillation column reactor which acts as a splitter taking a hydrodesulfurized light naphtha overhead along with the H2S produced in the reactor. A significant portion of the H2S is removed from the overhead accumulator/separator. The heavier fraction is hydrodesulfurized and removed as a bottoms. The bottoms and overheads are then fed to an H2S stripper wherein the remainder of the H2S is stripped from the desulfurized naphtha. The bottoms from the H2S stripper are then fed to a second reactor (either a standard single pass reactor or a second distillation column reactor). The removal of the H2S serves two purposes. It prevents the formation of recombinant organic sulfur compounds and allows for less severe conditions for the same sulfur removal while preventing hydrogenation of olefins.
As used herein the term xe2x80x9cdistillation column reactorxe2x80x9d means a distillation column which also contains catalyst such that reaction and distillation are going on concurrently in the column. In a preferred embodiment the catalyst is prepared as a distillation structure and serves as both the catalyst and distillation structure.